The present invention relates to the field of light emitting diode based lighting and more particularly to a distributed architecture for driving and controlling a plurality of LED strings having a single controllable power source.
Light emitting diodes (LEDs) and in particular high intensity and medium intensity LED strings are rapidly coming into wide use for lighting applications. LEDs with an overall high luminance are useful in a number of applications including, but not limited to, backlighting for liquid crystal display (LCD) based monitors and televisions, collectively hereinafter referred to as a monitor. In a large LCD monitor the LEDs are typically supplied in one or more strings of serially connected LEDs, thus sharing a common current.
In order supply a white backlight for the monitor, one of two basic techniques is commonly used. In a first technique one or more strings of “white” LEDs are utilized, the white LEDs typically comprising a blue LED with a phosphor which absorbs the blue light emitted by the blue LED and emits a white light. In a second technique one or more individual strings of colored LEDs are placed in proximity so that in combination their light is seen as a white light. Often, two strings of green LEDs are utilized to balance one string each of red and blue LEDs.
In either of the two techniques, the strings of LEDs are in one embodiment located at one end or one side of the monitor, the light being diffused to appear behind the LCD by a diffuser. In another embodiment the LEDs are located directly behind the LCD, the light being diffused by a diffuser so as to avoid hot spots. In the case of colored LEDs, a further mixer is required, which may be part of the diffuser, to ensure that the light of the colored LEDs are not viewed separately, but are rather mixed to give a white light. The white point of the light is an important factor to control, and much effort in design and manufacturing is centered on the need for a controlled white point.
Each of the LED strings is typically controlled by one of amplitude modulation (AM) and pulse width modulation (PWM) to achieve an overall fixed perceived luminance and, in the event of colored LEDs, color balance.
Each of the LED strings has a voltage requirement associated with the forward voltage drop of the constituent LEDs and the number of LEDs in the LED string. As the LED strings age, their voltage drops change, and furthermore, the voltage drops of the LED strings change as a function of temperature. In order to accommodate these requirements, the voltage output of the power source supplying power to a connected LED string must initially be set high enough so as to supply sufficient voltage over the operational life of the LED string taking into account a range of operating temperatures.
Ideally, separate power sources are supplied for each LED string, the power sources being set to exhibit a voltage output in line with the voltage drop across the associated LED string. Such a large plurality of power sources effectively minimizes excess power dissipation however the requirement for a large plurality of power sources is costly.
An alternative solution, which reduces the number of power sources required, is to supply a single power source for a plurality of LED strings. Unfortunately, since as indicated above different LED strings exhibit different voltage drops, such a solution further requires an active element in series with each LED string to compensate for the different voltage drops so as to ensure an essentially equal current through each of the LED strings of the same color connected to the single power source.
Utilizing a single fixed voltage power source for a plurality of LED strings thus results in excess power dissipation, as the power source is set to supply a sufficient voltage for all the LED strings over their operational life, which must be dissipated for LED strings exhibiting a lower voltage drop.
U.S. Patent Application Publication S/N US 2007/0195025A1 published Aug. 23, 2007 to Korcharz et al., entitled “Voltage Controlled Backlight Driver”, the entire contents of which is incorporated herein by reference, is addressed to a system for powering and controlling an LED backlight constituted of a plurality of LED strings receiving power from a single controllable power source. A control circuitry is operative to control the output voltage of the controllable power source responsive to an electrical characteristic of at least one of the plurality of LED strings.
The above solution provides a plurality of choices for closing the loop between an electrical characteristic, such as a voltage drop, of at least one of the plurality of LED strings, and a controllable power source. As the size of monitors grow the need for more and more LED strings is rapidly being foreseen. The solutions afforded by the above application are best suited to use in a single control circuitry, and thus are not suitable for large layouts where multiple control circuitry chips are required, each of the multiple control circuitry chips controlling a plurality of LED strings, where the multiple control circuitry chips and their associated pluralities of LED strings are arranged to operate in cooperation with a single controllable power source.